If we talk about positions then all of these players have at least one substitute, if not several ones apart from Mahrez. We know also that Mahrez didn't participate a lot in the previous season and was less effective comparing to two or three years ago.
Obviously having many players leaving the club will destabilise it but Manchester City know already that a transition phase will occur especially after they won everything especially Champions League title
Even so, Manchester City will not immediately become weak next season. Because Manchester City are very prepared for the transition phase they are currently experiencing, so there is still a possibility that Manchester City will continue to be strong next season, even though there might be a difference from last season when they won the championship. And in terms of substitute players, indeed it must still be there in order to anticipate unexpected things like you said that.Obviously having many players leaving the club will destabilise it but Manchester City know already that a transition phase will occur especially after they won everything especially Champions League title